Electrostatographic or electrophotographic processes involve a device one of the components of which includes a layer of photoconductive insulating material fixed to a conductive backing, i.e., a "photoconductor". Initially the surface of the photoconductor is uniformly, electrostatically charged over its entire surface, following which it is exposed to a pattern of light corresponding to an image to be reproduced. The charge on the surface areas impacted by the light of the image is thereby dissipated, leaving only areas not so impacted in the initially charged condition. The residual charge remaining on the surface of the photoconductor, therefore, conforms to the configuration of the pattern of light reflected from the image that is to be reproduced.
This latent electrostatic image can subsequently be developed or made visible by exposing it to finely divided, electrostatically attractable, particulate material. The material is drawn or attracted to the surface areas by the electrostatic charge thereon in amounts proportional to the magnitude of the charge in the electrostatically affected areas, thereby forming an image of the material being copied on the photoconductor.
The particulate material used to create the image, referred to in the industry as "toner", typically consists of a pigmented, thermoplastic resinous composition which can subsequently be transferred to a supporting substrate on which the document is to be permanently "fixed". Such transfer can be accomplished, for example, with the assistance of a corona discharge device which results in the creation of an electrostatic charge on the substrate, opposite in nature to the charge of the toner which forms the image on the photoconductor. Transfer of the toner image to the substrate by electrostatic attraction occurs when the substrate and the photoconductor with the image thereon are brought into close proximity with each other. The transferred image can thereafter be permanently fixed to the substrate by fusing the toner composition, using any of the several known methods.
Transfer of the toner to the latent electrostatic image takes place in a "toning zone" in which the photoconductor is brought into contact with a supply of the toner, either in the form of a two component "developer", i.e., magnetic carrier particles coated with toner composition, or in the form of a single component developer, consisting of toner which includes a magnetic component as part of the composition thereof. (Developer, as is used herein, covers the foregoing magnetic material. It does not cover non-magnetic single component developers.) At the point of contact, the developer is carried on a device often termed a "toning roller", and the developer is in motion relative to the photoconductor. The toning roller generally includes a core or roll having magnetic poles on its surface, and a shell in which the core rotates to move developer on the shell. The amount of developer carried on the toning roller at the contact point is of considerable importance in maintaining a high quality image. In this regard, if too little developer is carried on the toning roller, contact between the developer and the electrostatic latent image is reduced, resulting in an image having an undesirably light tone density. On the other hand, if there is an excess of developer present, such excess is forced outwardly along the longitudinal axis of the toning roller, where it can ultimately be lost from the ends of the roller, resulting in contamination of the electrostatographic device. Furthermore, excess developer can also accumulate at the entrance to the toning zone. This is sometimes called roll-back of the developer. This is undesirable since the area of the photoconductor in contact with the developer gradually increases as a consequence, resulting in a corresponding increase in the time of contact between the latent electrostatic image and the developer. This in turn causes an undesirable increase in the tone density of the image.
The problem described has long been recognized, and several remedial approaches have been suggested in an attempt to overcome it. For example, it has been proposed to use a doctor blade or "feed skive" adjacent to the roller, adjusted to provide just enough clearance between the edge of the skive and the surface of the shell to produce a coating of developer on the roller having the thickness necessary to assure that the toning zone will receive a supply of developer in the correct amount.
A disadvantage of developer flow control achieved through use of a feed skive, however, results from the sensitivity of results to the clearance between the skive and the shell. In this connection, a change of as little as 0.005 inch in the feed gap is capable of varying the tone quality of the image obtained. Proper adjustment of the gap, therefore, can be detrimentally affected by a skive whose surface is not perfectly straight, or by imperfections in the toner roll, for example, its non-circularity in the case of a rotating roll.
Another solution proposed has involved the use of a slotted feed plate positioned between the toner roll and the feed roller supplying it, the amount of developer passing through the slot determining the thickness of the coating on the toner roll, and therefore, the amount of developer available in the toning zone. Again, however, the dimensions are critical, and in this regard, as little as a 0.005 inch change in the slot width, or in the position of the feed plate relative to the toning roller, has a disproportionate influence on image quality.